Autoradiographic imaging of altered synaptic αβγ2 and extrasynaptic αβ GABA_A receptors in a genetic mouse model of anxiety

To image the possible alterations in brain regional GABA_A receptor subtype properties in a genetic animal model of human anxiety, mice heterozygous for the deletion of GABA_A receptor γ2 subunit (γ2^+/-) were studied using ligand autoradiographic assays on brain cryostat sections. The [³⁵S]TBPS binding assay was designed to reveal impaired GABA and channel site coupling shown to be more prominent in recombinant α1/6β3 than in α1/2β3γ2 or β2 subunit-containing GABA_A receptors expressed in HEK 293 cells. Increased GABA-insensitive [³⁵S]TBPS binding in the γ2 ^+/- mouse brains was evident in the cerebral cortex and in subcortical regions, the alterations being regionally similar to the loss of γ2 subnunit-dependent benzodiazepine (BZ) sites as revealed by [ ³H]Ro 15-4513 autoradiography. As the γ2 subunit protein is needed for synaptic clustering of GABA_A receptors, these results indicate that the extrasynaptic αβ3 receptors can be visualized in vitro as atypical GABA-insensitive [³⁵S]TBPS binding sites. The results suggest that GABA_Aergic synaptic inhibition is widely decreased in the brains of anxiety-prone γ2^+/- mice, while extrasynaptic GABA_A receptors are increased. These autoradiographic imaging findings further demonstrate the need to develop GABA_A receptor subtype-selective in vivo ligands to aid in assessing the contributions of various subcellular receptor populations in anxious and other patient groups.